256 



The agreement, in by far the biggest number of cases, of theory 

 and experience may be considered as an argument that the representa- 

 tion of molecules by our stereometrical formulae, as proposed by 

 and derived from the original hypothesis of Van 'tHoff and Le 

 Bel, allow as to give a fairly exact account of the most fundamental 

 geometrical properties of such carbon-derivatives. In this respect 

 these formulae may be conjectured as something more than mere 

 outlines, and as really corresponding in at least the most salient 

 features to the true configuration of the atoms in space. 



26. After these considerations we may now proceed to the 

 second part of Pasteur's law, connecting the molecular dissymetry 

 with the appearance of enantiomorphous, hemihedral crystal-forms 

 of the two isomerides, and examine more closely the truth of this 

 conclusion too. 



Since Pasteur discovered the occurrence of such enantiomorphous- 

 ly related, hemihedral crystal-forms in the case of both tartaric 

 acids, and since he confirmed his original view by the study of a 

 great number of their crystallised derivatives, the question as to 

 the necessary mutual dependence of both phenomena has been 

 discussed several times by different authors. 



While there seemed to be no justifiable doubt about the validity 

 of Pasteur's view that molecular dissymmetry is always the cause 

 of optical activity in the sense employed in the foregoing, serious 

 doubt has arisen in later times as to the correctness of Pasteur's 

 conclusion that molecular dissymmetry is necessarily connected 

 with the occurrence of enantiomorphous hemihedral crystal-forms. 



The reason of this is obvious. In fact, optical activity is a property 

 existing also in the amorphous state, in solution and in molten 

 masses. It is therefore a property which is directly and intimately 

 connected with the individuality of the single molecule and its 

 particular stereometrical configuration. The crystal-form however 

 is one of the many ways in which the symmetrical and periodical 

 arrangement of the crystal-molecules, or at least, of the crystal- 

 units, manifests itself. 



It may be, and more recent experience seems to support this 

 view, -- that the crystal-molecules are identical with the chemical 

 molecules ; or even that the conception of a single molecule in crystals 

 no longer holds. But also if this were true, the crystalline form, 

 while depending on the rather complicated arrangement of these 

 crystallographical units, is properly only a second-hand pheno- 



